1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing the same.
2. Description of Related Art
As typical in related art, solar cells are known that includes a photoelectric converter having a photoelectric conversion function, a collector electrode and a connector electrode respectively provided on a light receiving surface side and a back surface side of this photoelectric converter. Meanwhile, Japanese Unexamined Patent Application Publication No. Hei 4-223378 discloses solar cells that include a photoelectric converter provided with multiple through-holes and configured to extract carriers collected by a collector electrode on a light receiving surface side via through-hole electrodes formed inside these through-holes, for example.
FIG. 6 is a cross-sectional view for explaining a configuration of a conventional solar cell using a through-hole electrode. In FIG. 6, reference numeral 21 denotes a silicon substrate that has p-type or n-type conductivity. Through-hole 8 penetrating from a light receiving surface side to a back surface side is formed in part of silicon substrate 21. This through-hole 8 is formed by anisotropic etching using an alkaline solution. Through-hole 8 is formed so as to have a large cross-sectional area on the back surface side and a small cross-sectional area on the light receiving surface side.
Diffusion layer 23 is formed in the light receiving surface of silicon substrate 21 and in a wall surface of through-hole 8 by thermal diffusion of an impurity having opposite conductivity to that of silicon substrate 21, and thus forms a p-n junction to constitute photoelectric converter 2. Moreover, insulating film 3 is formed around through-hole 8 on the back surface of silicon substrate 21. This insulating film 3 is provided for electrically insulating through-hole electrode 5 to be described later from silicon substrate 21.
Further, light receiving surface collector electrode 61a is formed on the light receiving surface side of silicon substrate 21. Through-hole electrode 5 is formed, by vapor deposition, on a surface of the p-n junction formed on the wall surface of through-hole 8. This through-hole electrode 5 is formed continuously on insulating film 3 formed around through-hole 8 on the back surface of silicon substrate, on the wall surface of through-hole 8, and on light receiving surface collector electrode 61a. With this configuration, photogenerated carriers collected by light receiving surface collector electrode 61a are extracted from the back surface side via through-hole electrode 5.
Meanwhile, back surface connector electrode 71b is formed, away from through-hole electrode 5, on the back surface of silicon substrate 21.
According to the above-described conventional solar cell, it is possible to extract the photogenerated carriers collected by light receiving surface collector electrode 61a from the back surface side via through-hole electrode 5. Thus, there is no need to provide a connector electrode on the light receiving surface side and, therefore a light receiving area of the solar cell can be increased. Consequently, output can be increased.
In the conventional solar cell, through-hole electrode 5 is formed by vapor deposition. In vapor deposition method, first formation of thick through-hole electrode 5 is difficult. For this reason, in the conventional solar cell including through-hole electrode 5 formed by the vapor deposition method, it is difficult to reduce resistivity of through-hole electrode 5 and is therefore not possible to increase the output efficiently.
Meanwhile, another conceivable approach is to form through-hole electrode 5 by filling a conductive paste into through-hole 8, instead of the vapor deposition method. However, the use of the conductive paste requires thorough filling of conductive paste in the entire through-hole 8. Moreover, the filled conductive paste needs to be retained in the through-hole 8 for a predetermined period of time and to undergo a hardening or baking process. Nevertheless, if the conductive paste has low viscosity, it is impossible to retain the conductive paste inside through-hole 8 for the predetermined period of time. In this case, part of or the entire conductive paste may fall off from through-hole 8.
If part of or the entire conductive paste falls off from through-hole 8 as described above, resistivity of through-hole electrode 5 increases. This reduces output.
On the other hand, if highly viscous conductive paste is used for suppressing this problem, there is a risk that the conductive paste is not filled completely inside through-hole 8 and some space could be created therein. If the conductive paste containing such space undergoes a hardening or baking process, the resistivity of through-hole electrode 5 increases as a consequence or, moisture that enters the space may degrade product reliability. Meanwhile, if the conductive paste falls off from through-hole 8, a gap is generated between through-hole electrode 5 and the connector electrode. That gap may increase contact resistance.